Dynamic microphone



Patented Feb. 25, 1936 UNITED STATES PATENT OFFIQE DYNAMIC MICROPHONE Angeles, Calif.

Application October 25, 1933, Serial No. 695,113

1 Claim.

This invention relates to an improved dynamic microphone, and has for one of its principal objects the provision of a dynamic type microphone or speaker wherein extraneous circuit interruptions and interference are avoided.

One of the principal objects of this invention is to provide a magnetic microphone of the alternating current type which produces a strong output of current from a highly intensified magnetic m field whereby less amplification is required and corresponding distortion is minimized.

Another important object of the invention is the provision of a microphone wherein the magnetic field with relation to the coil attached to the diaphragm is removed from the center of the diaphragm to a circle or other points spaced between the center and periphery of the diaphragm wherein a greater smoothness of tone and sensitivity results.

20 Another advantage of the present invention is the elimination of lead wires for current required in the electro-magnetic or condenser plate types of transmitters and also the provision of a more compact apparatus which, at the same time, is

25 more powerful.

Another and still further important object of the invention resides in the provision of a microphone of the dynamic type wherein a thin nonmagnetic metallic diaphragm is employed which 30 metal can be selected with a view to a very slight range of expansion or contraction wherein a consequent evenness of tension results even under varying conditions of temperature and climatic changes,

Another important object of the invention is to provide, in a magnetic dynamic type microphone, a diaphragm which is so mounted as to be practically insensitive to wind pressure.

40 Other and further important objects of the invention will be apparent from the disclosures in the accompanying drawing and following specification.

The invention, in a preferred form, is illus- 5 trated in the drawing and hereinafter more fully described.

In the drawing:

Figure 1 is a vertical sectional view of the improved dynamic type microphone or speaker ele- 50 ment of this invention, the out being taken along the lines ll of Figure 3.

Figure 2 is a horizontal sectional view on the lines 2-2 of Figure 1,100king in the direction indicated by the arrows.

55 Figure 3 is a partial horizontal section on the lines 3-3 of Figure 1, looking in the direction indicated by the arrows.

As shown in the drawing:

The reference nmneral indicates generally the outer casing or body of the improved microphone of this invention, the same comprising essentially a cylindrical element as shown, having an open top member I2 and a bottom cap member 14, both of which are preferably screwthreaded in position, and which act to assist in 3Q retaining the inner elements of the device in desired relationship.

Mounted in the casing I0 is a plurality of permanent magnets of the horseshoe type as shown at 16, each magnet having one leg shorter than ,5 the other. The inner shorter legs are circularly grouped about a central supporting post 18 which is composed of non-magnetic material, prefer ably brass. It has been found in practice that eight magnets of this type produce very satis- 2O factory results, but it will be obvious that the number may be varied if found necessary or convenient. Suitable non-magnetic spacer members 20 are provided between the magnets, and proper ventilation is provided by means of air circulating openings or the like 22.

Mounted adjacent the top of the shorter inner ends of the grouped magnets I6 is an annular magnetic metal element 24 which, in addition to assisting to maintain the magnets in proper position by means of the retaining screw 26, also provides a single pole for the grouped magnets.

Mounted above the annular element 24 is the diaphragm 28 which is supported in proper position by suitable retaining elements at its periphery, and mounted on the lower face of the diaphragm is a coil 30 wound upon a hollow supporting core as best shown in Figure 1. It will be noted that this coil is removed from the center of the diaphragm for a considerable distance, 49 thereby providing a better action.

The ends of the coil 30 are mounted on suitable insulated connector bars 32 to which, in turn, are connected the ordinary contact current carrying wires 34. 4d

The housing is of non-magnetic material or of some material which provides magnetic insulation between the various magnets, and the magnets themselves are preferably composed of the usual magnet steel or a cobalt steel. 50

As the sound waves impinge upon the taut diaphragm 28, the attached coil 30 is caused to oscillate through the magnetic air-gap of the grouped magnets I6, and induced currents of varying intenslties are accordingly set up in the coil.

The field of magnetic lines of force is concentrated by the unique arrangement of the magnets and the rings 24, and the plate which rests on the long legs of the magnet, and accordingly a magnetic field of maximum intensity results which is out at its maximum point by the coil, and consequently the induced current in the coil 30 is much stronger than any which has been heretofore considered possible. This induced current is transmitted by means of the wires 34 direct to a proper radio amplifying means for sound producing purposes. Obviously, the arrangement may be reversed for sound transmitting purposes.

In practice it is found that the current delivered from this arrangement is of such volume that an amplification of not more than three stages ordinarily will suffice, whereas in the transmitters ordinarily and heretofore used, an amplification of at least five stages is required with consequent lessening of definition and with considerably more distortion.

It will be obvious that herein is provided a considerable improvement in this art, as, in transmitters used for sound recording on motion picture films, phonograph records or the like, it is objectionable to use direct current for the rea-- son that such a circuit picks up alternating current from adjacent sources or any extraneous circuit interruption, resulting in an alternating current hum or click in the receiver or speaker. For this reason, a purely magnetic type transmitter is preferable, as it produces an alternating current which is immune to other alternating currents. Heretofore, the magnetic type of transmitter has been faulty in that the output of current from a transmitter of weak magnetic field was insuflicient and required such a degree of amplification that the current variations were distorted, as also the receiving diaphragm vibrations, thereby injuring the quality of the sound reproduction.

Another advantage of this invention is the absence of feed wires for the current required in the electro-magnetic or condenser plate types of transmitters which adds much to the convenience of using this device. Only the two wires indicated in the drawing are required for feeding the amplifier. If desired, a third ground wire may be added.

Additionally, the greater current output possible by the use of this invention permits transmission over a greater length of circuit, eliminating the necessity of having the amplifier in juxtaposition with the transmitter, whereby considerable bulk and clumsiness of apparatus is avoided.

The alternating current does not have the loss in line voltage in transmission as does direct current.

It will be found that, when the microphone output is properly matched to the output transformer and the primary matched to the voice coil with the secondary matched to the amplifier tube or tubes, a very satisfactory and considerably improved transmission and recording results.

1 am aware that many changes may be made and numerous details of construction varied throughout a wide range without departing from the principles of this invention, and I therefore do not purpose limiting the patent granted hereon otherwise than as necessitated by the prior art.

I claim as my invention:

A microphone, comprising a series of grouped horseshoe magnets, said magnets having their like poles arranged about a central point, and a non-magnetic diaphragm adjacent the magnets, an induction coil on the diaphragm, semi-circular bussbars below the diaphragm and above the magnets, and connections between the bussbars, the induction coil and lead-in wires, one leg of each magnet being shorter than the other, with the shorter legs in the center, .a magnetic annular element mounted on the shorter poles and centrally of the diaphragm.

HARRY K. TARKINGTON. 

